1. Introduction
This invention relates to a process and materials for electroless metal deposition. More particularly, this invention relates to a novel electroless plating catalyst comprising a particulate having a catalytic coating uniformly dispersed in a liquid carrier and to a metal coating process using said plating catalyst. The plating catalyst and process are especially suitable for selective electroless metal deposition such as in the formation of electromagnetic interference (EMI) and radio frequency interference (RFI) shielding for housings for electronic equipment.
2. Discussion of Prior Art
Electromagnetic interference emissions are undesirable energy emissions within a frequency range of from less than 60 Hz to more than 1,000 MHz. Radio frequency interference (RFI) is the portion of EMI radiation in about the 0.01 to 1,000 MHz range.
EMI radiation is created by operation of many diverse forms of electronic equipment ranging from microwave equipment to home computers. The radiation occurs because electronic devices emit "noise" in the above frequency range that is picked up by other devices or by conduction through power lines that act as antennas. EMI radiation may interfere with other devices and has been known to cause such diverse problems as interference with police mobile radios, communication systems, scientific test equipment and cardiac pacemakers.
In recent years, the use of electronic equipment in the home and work place has grown rapidly with a concomitant increase in sources of EMI emissions. Additionally, most housings for electronic equipment are now fabricated from plastic rather than metal. Plastics are lighter, more versatile, easier to fabricate and less expensive than metal but do not possess the intrinsic EMI/RFI shielding capabilities provided by metal enclosures.
The Federal Communications Commission has published a series of regulations concerning standards for maximum allowable EMI emissions for electronic devices. The regulations, which became effective in October 1983, apply to all digital electronic products that use or generate frequencies between 10 KHz and 1,000 MHz. These regulations therefore include commercial, industrial, business, and home products such as computers, calculators, cash registers, electronic typewriters, video equipment, and electronic games. The regulations require that the electronics industry develop electronic devices which have electromagnetic compatibility (EMC); in other words, equipment which neither interferes with other devices nor is itself susceptible to interference.
One approach to limiting electromagnetic containment is the use of an EMI shield to contain the radiation. Containment requires special shielding materials, components, and structures which prevent generated energy from escaping and acting as a source of disturbance.
The effectiveness of electromagnetic containment is determined by the degree to which the field strength is attenuated as a result of reflection or absorption by the shielding material. Shielding efficiency is calculated as a logarithmic function of the ratio of unshielded EMI transmission to shielded EMI transmission and is expressed in decibels. Because of the logarithmic nature, an increase of 30 db in shielding efficiency for a given wavelength or frequency of electromagnetic radiation represents a 1,000 percent increase in the shielding efficiency of the coating. A coating with a shielding efficiency of 30 db, for example, eliminates 99.9% of the total EMI radiation. A 60 db coating eliminates 99.9999% of the total EMI radiation.
There are several shielding methods in commercial use for nonconductive materials. The method most often used involves a metallic coating applied over a plastic housing for the electronic device. An early publication showing the use of multiple metal coatings over a plastic housing is Engineering, 9, Dec., 1966, pp. 1026 and 1027. Methods for applying metallic coatings disclosed in this reference include galvanic deposition, spray coating, chemical metallizing and vacuum metallizing. Metal coatings include copper, silver, chromium, nickel, silver, gold, zinc, etc.
Metals are applied over housings for electronic equipment in a number of ways. For example, EMI shielding materials have been arc-sprayed (zinc) and painted with metal-containing paints (nickel) onto the electronic housings. Both of these methods have serious disadvantages. Arc-sprayed zinc is an effective EMI shield with attenuation to 120 db or more. However, zinc is toxic and expensive, the procedure is labor intensive, and the coating is prone to cracking and peeling. Conductive nickel paints are easier to apply than arc-sprayed coatings, but do not cover recessed areas, provide attenuation to only 20 to 60 db and often require multiple coatings.
Silver and copper conductive paints have also been used in the manufacture of EMI shielding. Silver is a good conductor, but is expensive and oxidizes. Copper conductive paints are easy to apply, economical, used with conventional equipment, are wear resistant and have good resistance to flaking. However, copper tends to oxidize which results in a loss of conductivity and a concomitant loss of shielding effectiveness
Other methods for applying metallic coating include cathode sputtering and vacuum metallizing. Such coatings show good conductivity and good adhesion, but require expensive equipment for application, are prone to microscopic cracking, can distort thermoplastics, require high power, are batch operations and are limited by part configuration.
Recently, interest has been generated in the use of electroless metals for EMI shielding. Electroless plating of surfaces for EMI shielding is shown in the prior art as early as 1967. Lordi, Plating, Vol. 54, p. 382, (1967), incorporated herein by reference, discusses the use of both electroless copper and electroless nickel as shielding materials. Lordi discusses electronic applications for electroless copper and electroless nickel specifically noting EMI shielding, teaches that electroless nickel can be used as an intermediate coating over copper to prevent corrosion and finally, that electroless copper can be protected by a coating of a second metal to prevent oxidation.
Recently, a number of publications have discussed the use of electroless metals for EMI applications. Plastics Technology,Vol. 27, June '81 p. 67, teaches the use of electroless metals as EMI shielding materials. Plastics World, Vol. 40, pp. 40-45, September 1982 states that electroless plating may be less expensive than many of the shielding processes now in use and can give comparable shielding performances. The economy of application of electroless plating for EMI shielding is demonstrated in a 1982 article in Industrial Finishing, Vol. 58, pp.100 to 101. Smoluk, Modern Plastics, September '82, pp. 48-51 cites several commercially available electroless plating systems for shielding applications. Smoluk reports electroless copper coatings with demonstrated SE values of 80 to 116 db, and electroless nickel coatings with SE values exceeding 45 db.
As discussed in the literature, both electroless copper and electroless nickel have been used in the electroless plating of plastic substrates. Both have advantages and disadvantages. Copper, with a relative conductivity of 1.0 (second only to silver with a conductivity of 1.05), has high shielding effectiveness. An additional advantage of copper is a relatively low cost. Disadvantages of copper are relatively low abrasion (wear) resistance and a relatively poor corrosion resistance with a strong tendency to oxidize which significantly reduces the shielding effectiveness.
Electroless nickel serves as a good paint base, has high wear resistance, stable electrical contact resistance, good solderability, and good corrosion resistance. Plastic Design Forum, November/December 1982, pp. 17-26, states that while electroless nickel is less conductive than electroless copper and therefore less effective as a shielding material, it possesses better corrosion resistance and may be preferable to electroless copper for EMI shielding applications, especially in severe environmental conditions. The major disadvantage to use of electroless nickel is its low relative conductivity of 0.20 or less. However, electroless nickel is relatively expensive and therefore, high cost is a disadvantage to the use of electroless nickel as a shielding material.
Disadvantages attendant to the use of electroless copper and electroless nickel separately as shielding materials are partly overcome by a dual layer of electroless copper overplated with electroless nickel. Such a dual layer is believed to be first suggested by Lordi (supra) in 1967. In 1983, Krulik, in Industrial Finishing, May, 1983, pp. 16-18, states that "the (electroless) copper's disadvantages are overcome by coating the copper layer with a thin layer of electroless nickel. The electroless nickel is deposited to protect the copper. The nickel's relative high cost is minimized by the thinness of the layer." A 1983 article by Hadju and Krulik in Plating and Surface Finishing, July, 1983, pp. 42-44, states that "a composite coating of electroless copper with a top layer of electroless nickel will combine the desirable characteristics of both. There is no degradation of the excellent shielding properties of electroless copper which can be adjusted in its shielding efficiency by varying its thickness. A relatively thin coating of electroless nickel provides corrosion resistance, paint adhesion, stable low electrical contact resistance, and other desirable properties and may be maintained at a constant thickness". A dual layer of electroless copper coated with electroless nickel is also disclosed in U.S. Pat. No. 4,514,486 incorporated herein by reference. This configuration utilizes the high conductivity of the electroless copper for EMI attenuation and the corrosion resistance of the electroless nickel to protect the copper against oxidation.
Briefly, electroless plating of plastics comprises immersing a part in a series of aqueous baths which both prepare the surface of the part for deposition and permit metallization. Following conventional pretreatment steps, a part to be plated is then immersed into a catalyst solution containing noble metals to render nonconductive surfaces catalytic to deposition of the desired plating metal. An example of a noble metal catalyst is disclosed in U.S. Pat. No. 3,011,920 incorporated herein by reference. The patent teaches treatment of the dielectric substrate with a colloidal palladium solution to render it catalytic to deposition of the dissolved metal.
Following catalysis, the part is then immersed into an electroless plating solution containing dissolved metals which, in contact with the plating catalyst, results in deposition of a coating of the metal onto the catalyzed surface.
Known procedures for electroless deposition of metal for EMI shielding are acknowledged by the art to provide superior coatings. However, one problem associated with their use is that the coating process is not selective. Coating is by immersion of the entire part to be plated into a liquid treatment solution--i.e., a colloidal catalyst solution followed by a metal plating solution. The result is that metal is plated over the entire surface of the nonconductor. Where aesthetics are important in the marketing of electronic components, a metal coated housing for the component is undesirable and typically, the industry paints the metal coating. This is a time consuming and wasteful step, especially where housings are most often molded in a desired color. For this reason, it would be desirable to have a selective process for plating only the interior of the housing without plating the exterior of the housing.
An attempt at selective plating of housings for EMI protection is disclosed in U.S. Pat. No. 4,670,306 incorporated herein by reference. In this patent, a process is taught comprising applying an adsorptive coating onto selected portions of an electronic housing where plating is desired. Selectivity is achieved by a masking procedure. This creates areas on the housing of differential adsorptivity. Thereafter, the housing is immersed in a catalyst solution and more catalyst is absorbed onto the absorptive coating than onto the balance of the housing thereby permitting selective metal deposition. In commercial practice, however, it has been found that selectivity is not adequate because of the required close control of all plating variables to obtain selectivity.
In published U.K. Patent Application Serial No. 2 169 925 A, incorporated herein by reference, another process for selective plating for EMI shielding applications is disclosed. In this process, a lacquer is used having suspended particles of metal which may be in the form of flakes, fibers, particulates and in one embodiment, commercially available silver coated glass spheres. The part to be plated is masked where plating is undesired, spray coated with the lacquer where plating is desired, the mask is removed and the part electrolessly metal plated selectively in a pattern conforming to the lacquer coating. The process of U.K. Application Serial No. 2 169 925 A is an improvement over that of above referenced U.S. Pat. No. 4,620,306 in that better selectivity is obtainable With fewer processing steps. However, a problem encountered with the process is the need to expose and treat metallic (catalytic) particles embedded in and sealed by the lacquer coating during coating and drying to form initiation sites for metallization. The steps of exposing and/or treating the particles prior to plating are costly and time consuming. Additional problems are encountered due to the high levels of metal loading in the lacquer which are normally in the order of 50% w/w. A high loading of metal is costly and the coating formed using this lacquer is rough in appearance as a consequence of the high solids content of the lacquer. Moreover, and possibly due to the rough surface, the metallic particles are poorly adhered to the substrate, flake off during processing and can fall into circuitry causing equipment problems and failures.